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Phil,
I just added some text on this to the web page description of RUC diagnosed variables. It reads as follows:
cloud fraction - (available in BUFR only) In the RUC, this is either 0 or 100% since non-zero cloud hydrometeor mixing ratios can only occur if the grid volume is saturated. (This characteristic is also found with MM5. The RUC uses MM5 mixed-phase cloud microphysics.) In the RUC, if there are any levels from the native grids with cloud water or ice mixing ratio > 10**-6 g/g, then the cloud fraction is set as 100% and 0% otherwise. This approach is different from the Eta model, where saturation exists at < 100% RH and the cloud fraction varies continuously from 0 to 100% based on a function of RH. Cloud layers in the RUC are defined as follows:
low - lowest 10 native levels (approximately lowest 50-75 hPa)
middle - above lowest 10 native levels and below 400 hPa level
high - above 400 hPa level
Basically, cloud fraction is an 'Eta-centric' variable, consistent with the cloud microphysics approach used in the Eta. It is possible that some klugey diagnostic of non-binary cloud fraction could be cooked up, but we don't have it now.
Steve, thanks for your comments also.
Stan
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